In general, printing and publishing processes, color proof is made using a proofer, an electrophotographic or ink-jet color printer, or a monitor of a personal computer (PC) or the like prior to main print processing. A technique required in such case pertains to color simulation that accurately reproduces colors of a printing machine used in main print processing.
The printing machine normally expresses colors by a subtractive process using four color materials, i.e., cyan (C), magenta (M), yellow (Y), and black (K), and these four colors are called process colors. A printer that performs color simulation or the like can reproduce mixed colors using the process colors of the printing machine by a color management technique. More specifically, profiles as databases that describe the device characteristics are used to absorb a color reproducibility difference between devices and, for example, device A can simulate the characteristics of another device B. Upon applying to color calibration of print processing, device A corresponds to a printer or monitor for simulation, and device B corresponds to a printing machine.
However, the printing machine often uses color materials other than the process colors called spot color inks in addition to the process colors. This is to meet the demand to use colors which are hardly reproduced by mixing the process colors, and to suppress cost. In order to simulate such spot color by a printer or the like, the spot color is expressed by a color value (Lab value) on a device independent color space (DIC space) such as an L*a*b* color space or the like on the basis of the name of the spot color, and the Lab value is converted into a value such as a CMYK value or the like on a device dependent color space.
No scheme for calculating a color obtained by mixing two spot colors (to be referred to as “composite spot color” hereinafter) has been established yet. For example, as a method of reproducing such composite spot color, a method of calculating one color value from the two color values of two spot colors on the DIC space using an arbitrary method may be used. However, this calculation can give an approximately correct value, but it cannot be an accurate calculation method. For example, if a cyan value upon converting a given spot color into a CMYK value is 90%, and that upon converting another spot color into a CMYK value is 80%, a cyan value upon compositing these two spot colors is 170% if their densities are simply added. However, the upper limit of the density value is 100%, and inconsistency occurs.
Even in case of the DIC space, since color spaces such as L*a*b*, XYZ, and the like, which are popularly used in color management, have nonlinear characteristics, it is impossible in principle to calculate color composition by means of addition, multiplication, or the like.